NBSIR 74-541

An Evaluation of Potentially

Useful Separator Materials for

Nickel-Cadmium (Ni-Cd) Satellite

Batteries

H. A. Baker. S. D. Toner, W. F. Cuthrell

Consumer Product Systems Section

Measurement Engineering Division

Institute for Applied Technology

October 1974

Final Report

Prepared for National Aeronautics and Space Administration

Goddard Space Flight Center

Greenbelt, Maryland 20770

NBSIR 74-541

AN EVALUATION OF POTENTIALLY

USEFUL SEPARATOR MATERIALS FOR

NICKEL-CADMIUM (Ni-Cd) SATELLITE

BATTERIES

H. A. Baker. S. D. Toner, W. F. Cuthrell

Consumer Product Systems Section

Measurement Engineering Division

Institute for Applied Technology

October 1974

Final Report

Prepared for National Aeronautics and Space Administration

Goddard Space Flight Center

Greenbelt, Maryland 20770

U. S. DEPARTMENT OF COMMERCE. Frederick B. Dent. Secretary

NATIONAL BUREAU OF STANDARDS, Richard W. Roberts. 0 if actor
-Bp'' •
Table of Contents

Page

Introduction 1

Materials ........ 1

Procedure for Apparent Density, Absorption,

Retention and Porosity 2

Procedure for Electrical Resistance 4

Procedure for Wettability 5

Procedure for Wicking 5

Procedure for Wet -Out Time . . 6

Procedure for Air Permeability 7

Discussion i 7

Observations 8

Table I 12

Table II 14

Table III 16

Table IV 20

Table V 25

Table VI

1 27

Table VII 29

Table VIII 30

Table IX 32

Figure 1 33

References 34

This study was undertaken for the Goddard Space Flight Center,

National Aeronautics and Space Administration, to develop and improve

testing procedures and testing techniques for direct electro-physicochemical evaluation of polymeric separator materials used in 6 ampere

nickel-cadmium (Ni-Cd) satellite batteries.

The intent of this study was to determine which tests, including

those in a proposed test method, were pertinent to the .evaluation of

separator materials, and to use these procedures to evaluate various

types of new materials being used in experimental batteries. In addition the procedures were designed to be used to evaluate other

potentially useful materials, without the need for fabrication and

extensive testing of actual batteries.

It was intended that these procedures would, if necessary, provide

a basis for evaluating structural changes in the materials that might

result in premature failure of batteries subjected to cycling tests;

and to determine the relationships between changes in properties to the

mode of failure and to service life; in order to establish criteria for

the selection of materials with optimum service characteristics.

After appropriate test procedures had been established using ten

different separator materials, they were used to evaluate the properties

of these same ten materials taken from experimental batteries after they

had been subjected to simulated use conditions. Subsequently, ten

additional materials were received for laboratory evaluation of their

potential use as separators, based on the previously obtained data. The

results obtained on all twenty of these samples were previously contained

in the NBS Technical Report No. 10956, "Evaluation of Separator Materials

Used in Ni-Cd Satellite Batteries," November 1972.

This report is concerned with and contains the results of the

laboratory evaluation, using previously established testing procedures,

of twenty-five separator materials, which had not previously been

studied. Consequently, these current tests were intended to determine

their potential suitability and probable efficacy if used in Ni-Cd

batteries

The methods currently used measure materials properties such as the

apparent density, absorptivity of the electrolyte, electrolyte retention,

porosity, electrical resistivity, wettability, wicking time, wet-out

time, and air permeability.

Materials

1he separator materials were in the form of sheets of non-woven

fibers. The fibers used in these materials were: nylon-6 (polycaprolactam) , polypropylene, grafted polypropylene, a polyethylene/polypropylene

blend, and aluminum oxide. One sample consisted of a filled plastic film

laminated to a non-woven asbestos substrate. The samples included in this

report were submitted to tins laboratory at various times between September

1972 and November 1975, and arc listed in table I.
All of the new materials were tested in the "as-received" condition,

that is in the same form in which the materials would be used to fabricate

batteries. Several of the materials had been subjected to surface treatments involving the application of wetting agents. Three of the

polypropylene materials had been subjected to grafting techniques in

attempts to chemically attach hydrophilic components to the polypropylene

fiber surfaces, in order to increase the wettability of the fibers.

Since evaluation of the properties of materials actually used in

batteries, and subsequently subjected to these test procedures, cannot

be accomplished without aqueous extraction of the electrolyte, it was necessary to subject the new materials to an aqueous extraction, using

a Soxhlet apparatus, in order to determine whether this process affected

the properties of the materials to the extent that a substantial change

due to the extraction would prevent evaluation, by these procedures, of

materials taken from cycled experimental batteries.

The first group of samples submitted for evaluation in this current

series were samples AA through SS~. Within this group there were five

pairs of samples in which the material used in each pair was identical

with respect to fiber content and weight per unit area, except that one

of the samples had been treated with a wetting agent. In each case there

was evidence of severe bleeding of the wetting agent from the sample

into the packaging materials in which they were received. Consequently,

the sponsor requested that these, and any other samples received at a- later date, of treated materials, as well as other selected samples, be

subjected to two twenty-four-hour aqueous extractions prior to conducting

any of the tests. This involved a variant in the normal procedure of

extracting the samples for twenty-four hours. In conducting the dual

extraction, the samples were allowed to air dry at room temperature

between the two extraction cycles. In addition, a number of selected

materials were also subjected to extraction with specified organic solvents prior to measurement of the absorption characteristics.

Procedures for Apparent Density, Absorption, Retention and Porosity 1/

These procedures involve the determination of the apparent density,

based on the dimensions and weight of specimens of the separator

material, the amount of electrolyte absorbed, the amount of electrolyte

retained after centr ifuging , and the porosity, calculated on the basis

of the volume of electrolyte absorbed.

Normally these tests are conducted on ten specimens. However, on

the basis of experience achieved in this laboratory on various experimental materials, it was found that some were not wetted, or incompletely

wetted by the electrolyte under the conditions of tests described below.

Consequently, when a new material was being subjected to a preliminary

analysis, only five specimens were used. If all of these specimens

wetted so that they were completely saturated, then five additional specimens were tested, and the average of all ten specimens was used to
- 3 -

characterise the material. In those cases in which the specimens were

incompletely wetted, it was normally possible to obtain some data on those

properties related to the absorption characteristics described in this

section. However, -it usually was not possible to obtain data on any of

the other properties, except air permeability. In some cases, extraction

resulted in the removal of all or part of the wetting agent so that

specimens from these materials did not wet completely when soaked in the

electrolyte. In such cases, the number of test specimens from the

extracted material was also limited to five.

For these tests, the specimens are cut from the sheet materials by

means of a steel die having a diameter of 2.54 centimetres (cm) (1 in).

The weight of each specimen is determined to the nearest 0.1 milligram

(mg) on an analytical balance. The thickness is obtained using a dial

micrometer gauge, graduated to read directly to 0.001 inch, and

with a finger lever and a 1.27 cm (0.5 in) diameter stainless steel

spindle. The anvil, or platform, on which the specimen is .placed must

be of sufficient size to accommodate the specimen. The gauge should

exert a spring pressure approximately equivalent to that of a 50 gram (g)

weight. To minimize the effects of compression on the separator materials

by the testing device, no additional weights are used on the gauge. The

nominal thickness is reported as the average of two readings, one taken

within two seconds after the spindle is placed on the specimen, and the

other after it has been in contact with the specimen for 3 minutes. The

specimens are then placed in a plastic container with 100 milliliters (ml)

of an aqueous, 31% solution of potassium hydroxide (KOH) and soaked for

at least 24 hours. Plastic beakers sealed with plastic film or plastic

bottles with screw lids are used. (Note: use of glass containers should

be avoided since reactions between the alkali and glass may effectively

dilute the electrolyte solution.) On completion of the soaking period,

the specimens are removed one at a time, and both sides blotted momentarily

on a fine grain ashless quantitative filter paper to remove excess KOH.

Blotting is accomplished by holding the specimen with stainless steel

tweezers and lightly touching each side to the filter paper. The specimen is then placed in a previously tared plastic weighing bottle, capped

immediately, and the weight of KOH absorbed is determined. The specimens

are placed between two stainless steel wire mesh discs in a vertical

position in a centrifuge, backed by a 0.3 cm thick stainless steel ring

and a piece of blotting paper, and then subjected to a centrifugal

force of 25 G's for two minutes. The specimens are then immediately

placed in previously tared, plastic weighing bottles, capped and weighed.

The amount of absorbed KOH retained is determined. Tne percent porosity

is determined based on the wet weight obtained just prior to centrifuging

.

Calculations

:

. . . ,3 dry weight Apparent density, g/cm = dry v ol ^me

KOH absorbed, % = wet vci * ht '.^ wei Sht

X 100

dry weicht
- 4 -

, , , ,3 wet weight - dry weight

KOH absorbed, g/cm = -f to 2 -—< &— dry volume

. . .

• o weight after centrifu^in^ - dry weight v , -

_

KOH retention, % = 5 j . ~. - 5 < 6— x 100 dry weight

% o£ absorbed KOH retained, % = \ °\

TM retained

% of KOH absoroed

wet weight - drv weight

Porosity, % = ~

J A ~ X 100

' dry volume

3

Note: 1.3 is average density, in gm/cm , of an aqueous solution

containing 31% KOH at ambient laboratory conditions.

When the specimens are removed from the electrolyte solution, after

having soaked for at least twenty-four hours, they are visually examined

and a subjective estimate is made of the approximate percentage of the

surface area that is thoroughly saturated. The dry materials are white

and opaque in appearance, while wetting to saturation causes their

appearance to become translucent. It is the percentage of translucent

area that is reported in table II, for specimens of both unextracted

and extracted samples. When the specimens did not become completely

saturated, the wetted areas were generally spotty and irregular, and in some cases may have been related to localized variations in the fiber

density within the individual specimens.

The data on the absorption characteristics are given in table III

for the "as -received" materials and in table IV for the extracted

materials.' It should be noted that even when the average area of the

wetted portions of the specimens is rather low, the amount of KOH

absorbed may be relatively high when compared to other materials.

Procedure for Electrical Resistance 2/

This procedure is used to determine the electrical resistivity of

the separator materials when saturated with an electrolyte. The resistance measurements are made by connecting an electrolyte-filled test

cell, in which the specimen is located between the anode and cathode

of the cell, to a universal bridge operated in the internal mode (1592

Hz). The test cell is designed so that the circular area of the specimen

undergoing test was 0.6S6 cm-. Five 2.54-cm square (1 in^) , or 2.54 cm diameter (1 in) specimens are used. The specimens are placed in 31°o KOH

solution for at least 24 hours prior to testing. On completion of the

soaking period, the test cell is assembled without a specimen, filled

with 31°o KOH solution and the resistance of the electrolyte measured to

obtain a blank reading. A similar measurement is made on completion of

each set of specimens, and the average of the two measurements is used

in calculating the resistance of the solution. A wet specimen is then

placed in the empty cell in such a manner as to permit measurements to

be performed at two different locations on each sample, for example, one
- 5 -

in the upper right quadrant and one in the lower left quadrant. Both

sides of the cell are filled with electrolyte and the resistance measured.

The average of these two measurements is used to determine the combined

cell and separator- resistance. Immediately after completing the resistance

measurement, the thickness of the wet specimen is measured using the

•gauge and technique previously described in the absorption test.

The electrical resistance characteristics are reported in table VI

as the specific resistivity.

Calculations

R

q

= cell resistance (blank) , ohms

R

g = cell resistance- with separator, ohms

R' = separator resistance, ohms *

R« = (R - R ) s o Separator specific resistivity, p , ohm-cm

R

P = — X A where: tw = wet separator thickness in cm A = area in cm

Procedure for Wettability

The universal bridge and test cell used in the electrical resistance

tests are also used to determine v/ettability of the separator material.

Wettability is measured as the time required for the electrical resistivity of the separator material to equilibrate. Ideally the specific

resistivity at equilibrium should equal that obtained on the pre-soaked

samples, and be achieved in a matter of minutes.

Five specimens are cut using a 2.54 cm square or 2.54 cm diameter

steel die. A dry specimen is placed in the empty test cell and the cell

connected to the bridge. One side of the cell is rapidly filled with 31%

KOH solution, and the timing of the experiment is begun as the second

side is being filled. The electrical resistivity is monitored and the

time for the resistivity of the system to remain constant for one minute

is observed. The time at the start of the one-minute stable period is

recorded as the wettability time. In those cases where a material does

not wet, the sample is allowed to remain in the cell for two hours as a means of verifying its non-wettability

The wettability test data are also presented in table VI.

Procedure for Kicking

Tins procedure is used to measure the rate of rise of electrolyte

solution up a vertically supported, partially immersed material. In

these tc>ts, due to the wide variation in wicking characteristics of the
- 6 -

materials the measurements were recorded either as "time to reach 6 cm,"

the ideal condition, or as "cm/24 hours" depending on which event occurred

first

.

In order to facilitate these tests a special holder, capable of

holding 5 specimens, was designed and built to provide a means for

uniformly positioning and immersing the specimens, and to measure the

height of the solution on the specimens. This device is illustrated in

Figure 1.

• The original intent of this procedure was to record the time required

for the solution to rise to a height of 6 cm. It is obvious from the

data given in table VII that several materials would require prolonged

test periods, if the solution could indeed travel to a height of 6 cm.

It was for this reason that all such experiments were discontinued after

24 hours

.

Since a test of this type is. sensitive to environmental conditions

such as changes in temperature, relative humidity and velocity of air

currents impinging on the specimen surface, the test should, be conducted

for a specified short period of time to reduce the effects of these

variables. For example, wicking action will stop when the rate of upward

movement of the solution equals the fate of evaporation of water in the

electrolyte from the specimen. It is believed that a short time test,

e.g., two to four hours, would provide an adequate means of assessing

variations in wicking characteristics of different materials. If it is

considered necessary to retain the longer test period, then it is obvious

that the tests should be conducted in a totally enclosed system.

Place five specimens, 0.64 X 8.9 cm (0.25 X 3 1/2 in), in the holder.

Place the holder in a tray of suitable size and fill the tray to a depth

of 10 mm above the lower ends of the specimens with 31 ?

o KOH. As an aid

to accurately distinguish the leading edge of the solution on the specimen,

a dye was added to the KOH solution. Phthalein purple indicator, used

in a ratio of 0.167 g per 100 ml, was found to produce a suitable, stable

color in the solution.

The results of these tests are given in table VII.

P rocedure for Ket-Out Time

This procedure is intended to determine the time required for a drop, approximately 0.1 ml, of electrolyte solution to be absorbed when

placed on the upper surface of a specimen of separator material.

In the process of developing this test it was noted that, as a result of the porosity of some of the materials, the solution tended to

seep through the specimen and spread by capillary action between the

specimen and supporting surface. It was found that placing the specimens

on three layers of nylon netting eliminated the capillarity problem.

The netting was held flat by affixing it to a polyethylene covered board

approximately 5 X 20 cm (2 X S in).
- 7 -

Five specimens 2.54 cm (1 in) in diameter are placed on the nylon

netting, and one drop of a solution of 51% KOI I , containing the same amount

of phthalein purple indicator used in the wicking tests, is placed in the

center of the specimen, the timer started, and the time required for the

entire specimen to become wetted, or until absorption is complete, is

recorded. Absorption is considered to be complete on disappearance of

the sheen normally visible on the surface of the liquid. In those

instances in which the entire specimen is not wetted on total absorption,

or on incomplete absorption after 1 hour, the area of absorption is

determined by means of a supported acrylic plastic disc having etched

concentric circles, which is placed over, but not in contact with the

specimen. The diameters of the circles are: 0.4S, 1.11, 1.59, 2.07 and

2.54 cm (3/16, 7/16, 10/16, 13/16 and 1 in), and are assigned numbers 1 to 5, respectively. In reporting the wet-out characteristics of a material, the assigned circle number or range is used.

The results of these tests are given in table VIII.

Procedure for Air Permeability

This procedure consists of meas-uring the time required for 300 ml

of air to permeate a .645 sq. cm (0.1 sq in) area of a specimen under

uniform pressure equivalent to 3.1 cm (1.22 in) head of water. A Gurley

Densometer, model 4110, with a 141 g (5 oz) inner cylinder is used for the

test. Measurements are made on ten samples from each material. These

measurements are made under standard laboratory conditions required for

textile fabrics, which are 21°C (70°F) and 6S°0 R.H.

The results of the tests are given in table IX.

Discussion

Importance of Separator Properties

It must be emphasized that the properties of the separator materials

in the original, "as -received" condition are of primary importance with

respect to their use in batteries. Any drastic or detrimental change in

properties as a result of aqueous extraction simply means that a realistic

evaluation of similarly extracted materials removed from experimental

batteries may be difficult or impossible to achieve. Measurement of

materials properties after extraction was necessary since all cycled

materials must be extracted to remove the electrolyte prior to retests

intended to detect changes in the materials that might be useful in

predicting the service life of the batteries.

The properties that appear to be most important in assessing the

suitability of a material for use as a separator material are as follows:

high electrolyte absorptivity, good electrolyte retention, low specific

resistivity, rapid wettability, and low resistance to air permeation.

Wicking characteristics and wet-out time seem to be most important with

respect to the initial filling of the battery with the electrolyte.
- 8 -

In these tests, however, one fallacy seems to have been detected

that had been deemed to be most important for a separator material, and

that is the need for a high degree of electrolyte absorption. In the

past, materials such as cellophane, and nylon in the form of film, woven

and non-woven fabric have been used, both alone and in combinations, as Ni-Cd battery separators. Both of these materials have very high

absorption capacities for many aqueous solutions due to the respective

presence of hydroxyl and amide groups in the polymers. However, both

materials are subject to degradation by hydrolysis when exposed to strong

alkaline solutions. The direct result of such degradation is the development of electrical shorts between plates with an eventual failure of the

battery

.

In this study, most of the separator materials were made using

polypropylene fibers, which, are very resistant to alkali hydrolysis.

However, since the polypropylene fibers are basically hydrophobic, they

do not wet-out when exposed to aqueous solutions. Consequently, efforts

have been made to treat these fibers with various types of wetting agents .

When considerations for retest of cycled materials are concerned,

it is obviously necessary that any wetting agent be resistant to' removal

by hot, aqueous extraction techniques. Consequently, on the basis of

work done to date, it appears that the overall criteria for polypropylene

materials can only be met if successful techniques can be developed for

the chemical grafting of hydrophilic, alkali-resistant, materials onto

the polypropylene fibers. On the basis of studies performed on these

types of materials, as well as previous studies comparing new and cycled

materials it appears that a high degree of electrolyte absorption may be

secondary to resistance to aqueous extraction and to chemical changes

during actual use, provided that other properties such as low specific

resistivity, low resistance to air permeation, and rapid wettability can

be achieved and maintained under actual service conditions.

Previous work on cycled materials had shown that the specific

resistivity and the air permeability were the properties that were most

indicative of changes in the separator materials. Therefore, when there

is little change in these characteristics over a period of simulated

use, it appears possible to predict the potential lifetime of a battery

by monitoring these two properties.

Observations

The rationale for use of non-woven, fibrous battery separator

materials is based in part on increased ion mobility as compared to that

obtainable with solid film separators. Non-woven material is particularly

suited to Ni-Cd systems since dendrite formation between battery plates

is not a problem.

Nylon has been used for a number of years as a separator material.

However, as mentioned above it is susceptible to hydrolysis. In these

tests, the nylon materials have been used primarily as standards, against

which so:.:e of the more alkali-resistant materials have been compared.
- 9 -

Polypropylene is highly resistant to alkaline hydrolysis and has

excellent electrical properties. However, it is non-polar and when

immersed, absorbs water to the extent of <0.05 percent. Consequently, it

exhibits very low wettability and electrolyte retention characteristics

in aqueous systems unless the fiber surfaces are treated, for example by

-oxidation, grafting, or coating with a wetting agent.

As stated previously, these samples were received at various times,

with samples AA through SS included in the first group.

Sample AA was a nylon material, described as non-wettable . In the

"as-received" condition, this material exhibited characteristics similar

to those of other non-wettable materials. However, after aqueous

extraction it exhibited all of the desirable characteristics for a separator material, and was comparable in these properties to previously

evaluated nylon materials. .

Samples BB through EE were described as non-wettable polypropylencs

,

differing from each other only in the weight per unit area. The former

three samples exhibited the usual properties of non-wettable materials.

Sample EE gave anomalous results in that it had all of the characteristics

of a useful separator material, and data were obtained on all properties

except for wet-out time on the extracted material. With the latter

exception, aqueous extraction had little effect on the properties as . compared to the "as-received" material. This sample, unlike the other

three of the set, appeared to consist of two thin, rather stiff, surface

layers, between which was a very thick, loose, and non-woven mat as the

center layer. The percent absorption of electrolyte was very high on the "as received" basis, and although there was a significant decrease

in absorptivity after extraction, the percentage of absorption was comparable to that noted for the nylon materials. This material, when

saturated with KOH was too thick to fit into the specimen receptical in

the centrifuge head without it being squeezed by. the wire mesh screens,

which undoubtedly accounted for the unusually low percentage of electrolyte

retention. It was also noted that when the specimens were placed in the

KOH solution for the absorption tests, and particularly in the conducting

of the wettability, wicking and wet-out tests, that the electrolyte seemed

to rapidly penetrate the outer surfaces, saturating the inner layer

prior to wetting of the surface layers.

Sample FF was described only as a polypropylene material. Its

properties, as determined in the various tests, indicated that it was a non-wettable material.

There were four pairs of polypropylene materials in which each pair

was identical except that one sample in each pair had been treated with

a wetting agent. These pairs included samples GG and HH, II and JJ , KK

and LL , and SS and RR in which the latter sample within each pair had

been treated. Similarly, samples MM and NiN were identical, with the

latter having been treated, but these materials had been produced from a blend of polyethylene and polypropylene fibers. Each of the wettable

samples in the five pairs exhibited good separator properties when

tested in the "as- received" condition. However., after two Soxhlet extractions with water, they generally exhibited the sa::.e characteristics of the
- 10 -

comparable non-wettable sample. The only exception noted was that

sample IIH still retained enough wettable characteristics to allow the

measurement of the specific resistivity. However, this value was considered to be too high to be useful in evaluating changes in the

material if it were used in an actual battery.

Sample PP was identified as a wettable polypropylene material of

Canadian origin. Although the amount of electrolyte absorbed by this

material was rather low by comparison with other wettable materials, its

overall properties were. excellent with respect to use as a separator

material. Aqueous extraction had no significant effect on its properties, and methanol extraction did not appear to change the properties

to any great extent. Chloroform extraction did result in a significant

decrease in the absorption characteristics.

Of the remaining samples, all were subjected to only one 24-hour

aqueous extraction, and none were extracted with organic solvents.

Sample TT was produced from polypropylene to which had been grafted an acrylic-based compound in order to improve wettability. Aqueous

extraction appeared to have no appreciable effect on the electrical and

absorption characteristics, except in the test for wet-out time. Its

properties both before and after extraction were similar to those of

sample PP, including a relatively low degree of KOH absorption.

Sample UU was identified only as a polypropylene material. However,

results obtained during the KOH absorption tests indicated that the

material was fully wettable and remained so even after aqueous extraction. In general, it behaved in a manner very similar to that noted

for samples PP and TT.

Samples W, WW, and XX were described as wettable polypropylenes

.

However, in the initial KOH absorption tests the "as -received" sample

of WW did not wet completely as did the other two samples. All three

closely resembled other non-wettable materials in their absorption and

electrical characteristics after aqueous extraction.

Sample YY was the only inorganic material received. It was a ceramic material produced from non-woven fibers of aluminum oxide bonded

by means of a butadiene-styrene elastorneric material. The sponsor

requested that prior to conducting the evaluation tests the binder be

removed by firing the material in a muffle furnace for 10 minutes at

700°C, and that in this state it should be considered as being in the

"as received" condition. The fired material was rather fragile and

difficult to handle. Test specimens subjected to the initial KOH

absorption test partially disintegrated in the electrolyte, negating any

chance of obtaining valid data on most of the subsequent test procedures.

Consequently, no further tests were conducted on the material.

Sample ZZ was a composite consisting of a zirconium oxide-filled

poly (phcnylene oxide) film laminated to a non-woven asbestos fiber

substrate. Electrolyte absorption was very low and appeared to bc- confined essentially to the asbestos layer. The plastic film appeared
- 11 -

to be slightly porous on the basis of a measurable electrical resistivity,

although it was essentially impervious to the permeation of air. Data

obtained on the Kicking and wet-out tests appeared to be due principally

to the presence of' the asbestos layer.

Sample J2 was a nylon material reputedly identical to a material

previously evaluated and reported on, identified as sample J. Sample J2

had been taken from a different lot of material. This sample exhibited

all of the characteristics expected of a nylon separator. However,

sample J2 was approximately 55 percent thicker than sample J although

both exhibited about the same weight per unit area. Consequently, a direct comparison between the two materials with respect to the apparent

density, amount of KOH absorbed in g/cm5

, and the percent porosity,

indicates that sample J2 was somewhat inferior to its counterpart.

However, all three of these properties are inversely proportional to

the thickness so that comparisons made on this basis indicated that the

materials were essentially identical. All other properties of the two • samples were essentially comparable and any minor variations could

easily be attributable to both variations in thickness and to fiber

density. '.

After the necessary specimens had been obtained from sample KK, the

remaining material was returned to the sponsor and subsequently submitted

to a commercial laboratory for the purpose of grafting a hydrophilic

material onto the polypropylene. The sample was saturated in a 5 percent

aqueous solution of methacrylic acid and then irradiated for 41 hours

with a Cobalt-60 source in an attempt to graft segments of poly (methacrylic acid) onto the fibers. The sample, when resubmitted to this

laboratory, for further testing, was designated as sample KK2. In the

"as received" condition it exhibited excellent properties for use as a separator. However, after aqueous extraction it exhibited the same

basic properties of the ungrafted, non-wettable sample KK. During the

extraction, the water in the reservoir flask contained copious quantities

of a white, flocculant precipitate. On completion of the extraction, the

water was evaporated to dryness and in addition to the precipitate a clear film was also noted on the walls of the flask. The clear film

apparently consisted of low molecular weight poly (methacrylic acid) which

is water-soluble, while the precipitate apparently consisted of a higher

molecular weight fraction of the polymer which is water-insoluble. It

seemed apparent that the radiation had resulted in the polymerization of

the methacrylic acid, but did not provide an adequate number of free

radical sites on the polypropylene fibers for sufficient grafting to occur to render the material permanently wettable. Some of the unextracted

material was placed in a dilute aqueous solution of KOH. The pll was monitored and found to decrease rapidly, indicating that the alkali was reacting with the free carboxylic acid groups in the polymer. This experiment indicated that if the material were used in a battery, it would

effectively dilute the concentration of the electrolyte. Dilution might

have some effect on the electrical characteristics of the electrolyte

with respect to the functional capabilities of the battery. Assuming that

suitable techniques of grafting poly (methacrylic acid) and the related

poly (aery lie acid) is later achieved, the above results indicate that the

grafted material should be washed in a dilute solution of KOH prior to its use in a battery.
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Table II. Degree of Electrolyte Saturation of

Battery Separator Materials

As Received Extrac ted

NBS Sample Hours in Saturated Hours in Saturated

Code KOH Area, % KOH Area, %

AA 91 50 24 100

BB 91 60 24 20

CC 91 50 24 10

DD 91 25 24 0

EE 24 100 24 4

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GG 94 6.0 24 20

HH 24 100 24 40

II 67 50 24 10

JJ 24 100 24 25

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NN 162 100

PP 24 100 24 100

RR 24 100 24 60

SS 24 10 24 15

SS 162 70

TT 24 100 24 100

UU 24 100 24 100

W 24 100 24 25

WW 24 50 24 25
- 15 -

Table II. Continued

Mo Dpppi' v r» rl Extracted

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- 27 -

Table VI. Specific Resistivity and V.'ettabi lity

of Battery Separator Materials

NBS Sample Specific Resistivity, ohm-cm a/ Wettability, minutes b/

Code As Received Extracted As Received Extracted

A A AA sJ 2. 10 c/ 3 .

6

BB sJ sJ ' £./ £/

CC sJ sJ c/ £/

DD sJ sJ £/ £/

EE 1.68 d/ 1 .70 d/ 1.3 52 .

8

FF c/

GG sJ £/ £/

HH 1.81 12.48 0.6 200 +

II £/ s_f £/ £/

JJ 1 .92 sJ • 0.6 sJ

KK c/ sJ £/ £/

LL 2. 05 c/ 0.8 SJ

MM £/ c/ £/ £/

NN 2 .06 2. 09 0.4 200 +

PP 3.50 3. 21 2 .

8

24 . o RR 2.11 c/ 0.9 £/

SS £/ sJ £/ £/

TT 3.54 3. 33 3.4 3/ . 0

UU 6.21 6.15 0.3 17.2

VV 1.96. £/ 10.9 £/

WW 27.27 f/ £/ 193.8 £/

XX 2.28 c/ 3.9 £/

zz 9.52 9.56 62.8 56.4

J2 1.49 g_/ 1.49 3.0 13.0

KK2 1.86 c/ 2.7 £/

e/

a_/ Average of 10 specimens

b/ Average of 5 specimens

£/ Sample would not wet, the test

could not be performed
Table VI. Continued

- 28 -

d/ Sample is squeezed when placed in the cell for

test and does not recover before thickness measurement is made. This difference in thickness would

make the resistivity answer lower than the one reported

.

ej Test discontinued after 200 minutes,

f/ Specimens not completely wet.

gj Average of 20 specimens. -
- 29 -

Table VII. Kicking of Battery Separator

Materials a/

NBS Sample As Received Extracted

Code Time to reach Tine to reach

6.0 cm cm/ 24 hrs. 6.0 cm cm/ 24 hrs

min sec min sec

AA - b/ - c/

BB b/ - b/

CC - b/ - b/

DD - b/ - b/

EE - 3.0 -

' 1.9

"

FF b/

- b/

GG b/ - b/

HH 6 44 - 0

II " - b/ -. - b/

JJ 4 32 - - - 0

KK - b/ - b/

LL 4 8 - - - 0

MM b_/ - b/

NN . • 4 15 - ^| - 0

PP 72 0 - - 3.6

RR . 8 5 - - - 0

SS b/ b/

TT 16 .22 - - 3.2

UU 2 39 - - 5.1

W 9 7 - - 0

\m - 1.0 - 0

.XX 3 19 - - 0

• ZZ - £/ - £/

J2 411 30 SJ

KK2 50 3 - - 0.4

a/ Average of 5 specimens

b/ Sample would not wet, test could not be performed.

c/ Time for test was greater than 7 hours but less

than 24 hours.
- 30 -

Table VIII. Wet-Out Time of Battery

Separator Materials a/

NBS Sample

Code

As Received Extracted

Time Circle Range Time Circle Ranee

Til 1 Tl illj.ll cpp ^ \- s-* min sec

AA b/ 3 26

BB b/ b/

CC b/ b/

DD b/ b/

EE - 1,6 2 c/ b/

FF b/ b/

GG y b/

HH - 19.4 3 - : 4 b/

II b /

• b/

JJ - 44.2 5 b/

KK b/ b/

LL - 27.4 4 - 5 b/

MM b/ b_/

NN 2"2.4 4 - 5 b/

PP 10 11 5 1/2 - 4 b /

RR - 3.0 5 - 4 b/

SS - b/ b/

TT - 8.4 5 - 4 by

UU 5.6 3 1/2 - 4 b/ vv 9.2 3 - 3 1/2 b/

WW 2 33 2 - 4 1/2 b/

XX 11.2 4 1/2 - 5 b/

zz 6 48 d/ 4 - 5 1 49

J2 9 9 2 - 3 8 22

KK2 3.5 2 - 2 1/2 b/

2 1/2 - 3

2 1/2-4

2-2 1/2

a/ Average of 5 specimens.

b/ KOFI in contact for one hour and specimen showed no absorption

or spreading.

c/ KOit penetrates through the surface layer with very little spreading but

spreads in fibers under surface to a range of 4 to 4 1/2 in an average of

25 seconds.
- 31 -

Tabic VIII. Continued

d_/ The results given are with the KOH in contact with the asbestos

side of the sample. One specimen was tested with the KOH in contact

with the coated side and the results were: as received 32 min 20 sec range 3; extracted 40 min range 3.
- 32 -

Table IX. Air Permeability of Battery

Separator Materials a/

NBS Sample No. of Seconds to Pass 500 ml of Air

Code Specimens As Received Extracted

Avg. Avg.

AA 6 . 13.8 11.0

EE 6 2.1 1.9

HH 6 10.9 6.5

JJ 6 15.6 11.4

LL '6 25.6 21.0

NN 6 12.8 7.-7 •

PP 10 5.0 4.

1

RR 6 13.6 8.1

ss 6 8.7 8.6

TT 6 1.9 1.4

Mil uu 6

A 1 4 . 1 4 . 0

w 6 3.4 1.1

WW 6 3.0 3.4

XX 6 2.6 2.4

zz V

J2 20 2.4 2.4

KK2 10 19.0 16.6

a/ Some samples were not subjected to this test at the

request of the sponsor.

b_/ One specimen tested, the result was SS hours to pass

approximately 175 ml of air.
- 33 -

SPECIMEN,' HOLDER

SPRCER

SPECIMEN]

STAlVJLtCS

STEEL Pi 10$

a:

3"

FROJT

NJVLOVJ SCREWS

6 cm SCALE, > 1 mm U k) ITS

lO^mTo LIQUID

PERSPECTIVE VIEW

RPCK BNJD HOLDER

Figure 1. Rack and Specimen holders for vicking test. All

materials of acrylic plastic sheet unless otherwise

noted. Pins on individual holders are offset to the

right to facilitate re-.ii.ng of scale.
- 34 -

References

Adapted from: "Characteristics of Separators for Alkaline Silver

Oxide Zinc Secondary Batteries," Ed., Cooper, J.E. and Fleisher, A.,

report from the Air Force Aero Propulsion Laboratory, Wright-Patterson

Air Force Base, Ohio.

Ibid., "Electrical Resistance: Alternating Current Method,"

Chapter 4, Section b.
U.S. OEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO.

NBSIR74-541

2. Gov't Accession

No.

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

An Evaluation of Potentially Useful Separator Materials

for Nickel-Cadmium (Ni-Cd) Satellite Batteries

5. Publication Date September 1974

6. Performing Organization Code

446.04

7. AUTHOR(S)

H.A. Baker, S.D. Toner, W.F. Cuthrell

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

. . DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No. 4460416

11. Contract/Grant No,

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

National Aeronautics and Space Administration

. Goddard Space- Flight Center

Greenbelt, Maryland 20770

13. Type of Report &: Period

Covered

Final

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.)

An evaluation intended to determine the potential suitability and probable

efficacy of a group of separator materials for use in nickel-cadmium (Ni-Cd)

satellite batteries was carried out. These results were obtained using test procedures established in an earlier evaluation of other separator materials, some of which had also been used in experimental battery cells subjected to simulated

use conditions . .

The properties that appear to be most important are: high electrolyte

absorptivity, good electrolyte retention, low specific resistivity, rapid wettability and low resistance to air permeation. Wicking characteristics and wet-out

time seem to be more important with respect to the initial filling of the battery

with the electrolyte.

While the properties of some of these materials indicate that they would be

satisfactory, no specific conclusions can be drawn as to their actual effectiveness without further testing after they have been subjected to simulated use

• conditions in experimental cells.

17. KEY U'ORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons

)

Battery separators; nickel-cadmium batteries; satellite batteries;

separator materials

18. AVAILABILITY YJT Unlimited 19. SECURITY CLASS

(THIS REPORT)

21. NO. OF PAGES

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